This proposal outlines studies to examine the effect of chronic hyperglycemia on insulin secretion, action, and messenger RNA levels in normal rats using a new technique for chronic glucose infusions. We hypothesize that chronic B cell stimulation results in a functional change characterized by selective glucose insensitivity, and have found this response to be lost in normal rats after 48 hours of plasma glucose levels 40 mg/dl above normal. This method will be used to examine the effects of different levels of glycemia for different periods of infusion on plasma insulin, glucagon and glucose concentrations, and on pancreatic insulin content. The in vivo and in vitro B cell responsiveness to a variety of secretagogues will be measured at several points to map out the progression of the A and B cell defects. The reversibility of the B cell abnormalities will also be examined at several points following discontinuation of the infusion. Similar methods will be used to assess the effect of chronic hyperglycemia on insulin sensitivity using the in vivo glucose clamp and in vitro techniques. The effect of chronic hyperglycemia on insulin synthesis will also be studied by measuring proinsulin messenger RNA levels at several points during the infusion and at different levels of glycemia.